Hitherto, a polyvinyl alcohol film has been produced by dissolving a polyvinyl alcohol-based resin in a solvent such as water to prepare a stock solution, subsequently forming a film by a solution-casting method (hereinafter, referred to as a casting method), drying the film using a metal heating roll or the like, and, if necessary, subjecting it to a thermal treatment. The polyvinyl alcohol film thus obtained has been utilized in a large number of applications as a film excellent in transparency and dyeing property, and a polarizing film is cited as one of useful applications thereof. Such a polarizing film has been used as an elemental constituent element of liquid crystal displays and in recent years, its use has been extended to displaying equipments for which high definition and high reliability are required.
Under such circumstances, as the screen size of a liquid crystal television set or the like is enlarged, there has been desired a polarizing film more excellent in polarization performance, especially in-plane uniformity of polarization performance than conventional ones. In order to obtain a polarizing film excellent in in-plane uniformity of polarizing performance, it is important that the polyvinyl alcohol film to be used as a raw film of a polarizing film is optically homogeneous and, in particular uniformity of in-plane retardation value is important. As one solution, there is proposed a polyvinyl alcohol-based polymer film wherein the film width is 2 m or more, the difference of retardation values between two points which are 1 cm apart from each other in a width direction is 5 nm or less, and the difference of retardation values between two points which are 1 m apart from each other in a width direction is 50 nm or less (for example, see Patent Document 1). Moreover, there is proposed to use a polyvinyl alcohol film wherein the water content of the film at the time when it is peeled from a cast base material is set at less than 10% by weight (for example, see Patent Document 2).
Incidentally, in the following, the “retardation” means an “in-plane retardation value” unless otherwise stated.
However, even by these disclosed technologies, the retardation of the resulting polyvinyl alcohol film is not constant and unevenness may occur in a polarizing film produced using the polyvinyl alcohol film as a raw film. With regard to the polyvinyl alcohol film of Patent Document 1, in the case where the film width is 2 m or more, there is several tens nm of retardation difference between two points on the film and hence the film cannot meet recent demand that an optically homogeneous film having a width of 3 m or more be obtained. Furthermore, since the retardation itself is large, optical design of a whole polarizing plate becomes complex.
In general, with regard to a polyvinyl alcohol film, there appears a tendency that the retardation increases at the both ends and decreases at the central part. One cause thereof is that dehydrative shrinkage occurs during a drying step and a thermal treating step of the film and thus stress is apt to be imparted to the both end parts of the film. The stress is apt to be generated as the water content of the film decreases and the stress increases in the step where the water content becomes 20% by weight or less, particularly in the step where the water content becomes 10% by weight or less.
Moreover, as Patent Document 2, in the case where the water content of the film at the time when it is peeled from the casting base material is less than 10% by weight, unevenness of the retardation increases before the peeling step.
The unevenness of the retardation in a width direction becomes remarkable as the film is widened and, in the currently used polyvinyl alcohol film having a width of 2 m, unevenness of the retardation of about 10 nm is present between both ends and the central part. As the width of the film is widened, it becomes difficult to flatten the retardation in a width direction. This is a reason why an optically homogeneous film having a width of 3 m or more cannot be produced by the conventional production processes.
Patent Document 1: JP-A-2002-28939
Patent Document 2: JP-A-6-138319